This invention relates to a flow control valve suitable for use in a high-pressure, small-flow fluid pressure system which, such as a brake device for an automobile, which is operated with an especially low-viscosity hydraulic fluid.
FIG. 5 shows a prior art flow control valve in which a spool 2 is slidably mounted in a sleeve 1.
The sleeve 1 is formed with an inlet port 3 at one side thereof near its top end and with an outlet port 4 in its bottom end. An annular groove 5 is formed in the inner peripheral surface of the sleeve 1 so as to communicate with the inlet port 3 and extend over the entire circumference of the inner peripheral surface.
The spool 2 is also formed in its outer periphery with an annular groove 6 extending over the entire circumference thereof. It is further formed with a plurality of radial passages 8 interconnecting the annular groove 6 with a fluid passage 7 extending through the center of the spool 2.
The fluid passage 7 has its top end open and communicates with an upper chamber A formed in the sleeve 1. Its bottom end communicates with a lower chamber B of the sleeve 1 through an orifice 9.
A compression spring 10 serving as a biasing means is mounted in the lower chamber B in the sleeve 1 to exert an upward force on the spool 2.
In such a conventional fluid control valve, when the fluid pressure differential between both ends of the spool 2 is smaller than a predetermined value, the spool 2 will be pushed up by the spring 10 to a position where the annular grooves 5 and 6 communicate with each other. Thus in this state, fluid flows from inlet port 3 through annular groove 5, annular groove 6, radial passages 8, fluid passage 7, and orifice 9 and into the lower chamber B in the sleeve 1 and is discharged through the outlet port 4.
When the pressure of the fluid flowing into the upper chamber A in the sleeve 1 through the inlet port rises and the pressure differential between both ends of the spool 2 increases, the spool 2 will descend compressing the spring 10 under the pressure differential. As a result, the annular groove 6 will move downwards out of communication with the annular groove 5, thus breaking communication between the inlet port 3 and the upper chamber A.
When the inflow of fluid is cut off and the fluid in the upper chamber A begins to flow out through the orifice 9, the spool 2 will be pushed up by the spring 10, restoring the communication between the annular grooves 5 and 6.
By repeating this operation, the pressure differential between both ends of the spool 2 will be kept equal to the biasing force of the spring 10 divided by the sectional area of the spool. Thus, the flow rate through the orifice 9 will be set at a predetermined value because it is established by the pressure differential.
If such a conventional flow control valve is used for the brake device of an automobile, since it would operate in a low-viscosity, high-pressure line, when the annular groove 5 in the sleeve 1 and the annular groove 6 in the spool 2 are displaced from each other, fluid leakage through a gap formed between the walls separating the grooves 5 and 6 would tends to occur to such a degree as not to be negligible. Thus, the displacement between the annular grooves 5 and 6 for the spool to attain a balanced position will grow so large that the position of the spool 2 will be unstable. This makes it necessary to use an urging means having a longer stroke.
Also, due to the fluid leakage, the actual flow rate may deviate from the target flow rate.
On the other hand, if oppositely disposed lateral holes are formed in the sleeve instead of the annular groove to reduce such fluid leakage, though the leakage can be reduced, the unbalanced force acting on the spool 2 in a diametrical direction will increase due to high fluid pressure, making it necessary to use a large spring in order to exert a correspondingly large the biasing force.
Further, since the intended flow rate is extremely small, if the biasing force is large, the orifice will have to be extremely narrow.